An AC induction motor is a type of electric motor for powering driven equipment. They are robust, reliable, and inexpensive and standardized throughout the world. Starting an AC induction motor requires a careful compromise between the cost of the motor starter apparatus, starting performance and the electric supply authorities regulations.
A basic method of starting an electric motor is by closing a contactor to allow the motor to start at full voltage as a direct-on-line starter. Although it is a compact and inexpensive method, it is not the best method to use. Direct-on-line starting is marked by inrush current surges of six to eight times the motor's full load ampere value, resulting in electrical surge transients as well as mechanical strain on driven equipment. It results in a fast breakaway and acceleration up to full speed in an uncontrolled fashion.
The effect upon a hydraulic pump is mechanical stress applied on the rotating components followed by surges in the hydraulic system. This may include a high initial flow rate causing a vacuum to be drawn on the suction side, pump cavitations and pressure pulsations at the pump discharge, Similar effects are found with mechanical loads such as conveyers (driven by a motor) which when subjected to sudden jerks or severe applications of torque, may lead to load displacement, conveyor belt slippage or breakage.
Equally, when stopping the rate of deceleration is totally uncontrolled, this leads to further mechanical stress on pipelines for pumping applications, mountings and check valves from the inertia of the flowing fluids. It also produces pressure surges in the hydraulic circuit leading to pipeline rupture and leakage of product.
To maintain continuity and quality of the supply of electricity, electric supply utilities apply service rules and regulations that limit the kW size of motors that can be connected with the direct-on-line method of starting which may affect other electric power consumers.
For large motors, from 35 kW up to 5000 kW, the supply authorities may require the application of reduced inrush current starting to limit the starting current surge to a low value.
Although there are numerous variations of motor starting with a reduced voltage, the prior art auto-transformer starter is the most efficient and common methodology for starting a large motor, because it is the only reduced voltage starter that can provide maximum starting torque with minimal line current; this is due to the transformation ratio of the number of turns of the primary and secondary windings of the auto-transformer.
Other known non-electronic types of reduced voltage starters are the primary reactor and primary resistance starters. Both primary reactor and primary resistance starters have similar starting characteristics and disadvantages as the direct-on-line starting methodology which causes transient current and torque peaks when changing from reduced voltage to full fine voltage.